py3: fully fix bundlepart.__repr__ to return str not bytes

My previous fix did not fully fix the issue: it would attempt to use
%-formatting to combine two strs into a bytes, which won't work. Let's just
switch the entire function to operating in strs. This can cause a small output
difference that will likely not be noticed since no one noticed that the method
wasn't working at all before: if `id` or `type` are not-None, they'll be shown
as `b'val'` instead of `val`. Since this is a debugging aid and these strings
shouldn't be shown to the user, slightly rough output is most likely fine and
it's likely not worthwhile to add the necessary conditionals to marginally
improve it.

Differential Revision: https://phab.mercurial-scm.org/D8091

mercurial/bundle2.py

@@ -1013,10 +1013,9 b' class bundlepart(object):'
         self._generated = None
         self.mandatory = mandatory
 
-    @encoding.strmethod
     def __repr__(self):
-        cls = b"%s.%s" % (self.__class__.__module__, self.__class__.__name__)
-        return b'<%s object at %x; id: %s; type: %s; mandatory: %s>' % (
+        cls = "%s.%s" % (self.__class__.__module__, self.__class__.__name__)
+        return '<%s object at %x; id: %s; type: %s; mandatory: %s>' % (
             cls,
             id(self),
             self.id,